It uses preset images, but as soon as you made screenshots (via F12 usually), they will be used. At least that's what I experienced with Arkham Asylum
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ZommuterDec 9 '11 at 10:11

@TobiasKienzler It definitely also uses your own screenshots. I took a random one of a dead bunny in skyrim (I'm a monster, what can I say) and it appeared as my background to guilt trip me. I'd add your comment as an answer.
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AlexDec 9 '11 at 15:07

3 Answers
3

The background images for a game in your library (and also for the game's store page!*) are taken from the publisher / store page † and your gallery of recent screenshots.‡

* If no background image was specified by whoever made the store page, it simply chooses the right-most screenshot from the game's gallery. Methodology: Looked at very old games on steam, then some new / popular games. Results held up to prediction on every case attempted.

† Images are taken from the store page only in the case that no images have specified by the publisher, that is.

‡ Methodology: I have some old pictures of The Guild 2 in my gallery, about half a year old. After clicking back and forth for new images in my library about 30 times, they've yet to show up as a background for the game.

To see if a newer picture would appear as a background, I loaded the game, and took a screenshot and uploaded to the cloud. It showed up the first attempt.

Similarly, taking another screenshotwithoutuploading to the cloud also resulted in that image being posted as a background on first attempt.